1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, and a facsimile machine. More particularly, the invention relates to an image forming apparatus that transfers a toner image on an intermediate transfer member or a latent image carrier to a recording medium using a transfer bias applied by a transfer unit.
2. Description of the Related Art
In an image forming apparatus with an electrophotographic system, electrostatic latent images obtained by forming optical image information on a latent image carrier such as a photosensitive element uniformly charged in advance are visualized by toner from a developing unit. The visible images are transferred on a recording medium such as a transfer paper sheet directly or via an intermediate transfer member such as an intermediate transfer belt and fixed onto the recording medium, whereby image forming is performed. In most of such image forming apparatuses a direct current transfer bias is applied using a transfer unit at the time of transfer from an image carrier such as the photosensitive element or the intermediate transfer member to the recording medium.
Recently, as a recording medium for the image forming apparatus, various types of sheet of paper such as a sheet with an expensive-looking leather-like pattern or a Japanese-paper-style sheet have become increasingly in use. Some of such recording media have some roughness due to emboss processing, for example, on the surface thereof with the purpose of creating an expensive look. When toner images are transferred on such a recording medium, the toner hardly adheres to the recesses on the surface of the recording medium compared with the protrusions thereon. Accordingly, when toner images are transferred on a recording medium with relatively large surface unevenness, the toner cannot be sufficiently transferred onto the recesses, whereby the image density on the recesses is likely to be relatively low compared with that on the protrusions. As a result, an uneven density pattern following the pattern of unevenness on the surface of the recording medium readily occurs in the images.
As a method to improve the defective transfer to the recesses on the surface of the recording medium described above, a method to use a transfer bias in which an alternating current component is superimposed on a direct current component, and the polarity thereof changes with time (hereinafter, referred to as the superimposed transfer bias) has been known and proposed in Japanese Patent Application Laid-open No. 2006-267486, Japanese Patent Application Laid-open No. 2008-058585, Japanese Patent Application Laid-open No. 9-146381, and Japanese Patent Application Laid-open No. 4-086878, for example. By switching the transfer mode between the direct current transfer mode and a transfer mode in which the alternating current component is superimposed on the direct current component (hereinafter, referred to as the superimposed transfer mode) depending on the type of recording medium to be fed into the image forming apparatus, appropriate transferability can be obtained for various types of recording media including such a recording medium with relatively large surface unevenness.
It is known that transferability to recording media depends on a deterioration state of toner. For example, when an image having a low image area ratio is consecutively output, a small amount of toner is supplied to and discharged from the developing unit, thus a large amount of toner that has been stirred for a long time remains in the developing unit. The toner that has been damaged due to such stirring for a long time, of which outer additives are buried in or isolated from the toner, deteriorates flowability of developer or changes charge properties of the toner. As a result, transferability is deteriorated, whereby sufficient transferability can be hardly obtained.
As a method to improve the low transferability due to the deterioration of toner as described above, a method to replace the toner in the developing unit with new toner replenished while forcibly consuming the deteriorated toner in the developing unit has been known and proposed in Japanese Patent Application Laid-open No. 2008-216601, Japanese Patent Application Laid-open No. 2006-47651, and Japanese Patent Application Laid-open No. 2007-108623, for example.
As a result of study, the inventors of the present invention have found that the effect of the deterioration of toner on the transferability depends on the existence of unevenness on the surface of the recording medium. In other words, the effect of the deterioration of toner on the transferability varies depending on whether the direct current transfer bias or the superimposed transfer bias is used. Specifically, when toner is transferred to a recording medium with roughness using a superimposed transfer bias in a superimposed transfer mode or the like, the effect of the deterioration of toner on the transferability is significant, whereby transferability when the deteriorated toner is used is remarkably deteriorated. Accordingly, the deterioration state of toner with which the transferability is permissible in the direct current transfer mode may cause remarkable deterioration of transferability exceeding tolerance in the superimposed transfer mode. This is probably because deteriorated toner cannot follow the change of the bias with time in the superimposed transfer mode, whereby the toner within the transferred field cannot exhibit an intended behavior.
In view of the above, there is a need to provide an image forming apparatus capable of improving transferability when a superimposed transfer bias is used even if deterioration of toner in a developing unit of the image forming apparatus has progressed.